1. Technical Field
The present invention relates to methods for surface treating an engine valve made from titanium or titanium alloy in order to improve abrasion and impact resistances, and also to engine valves treated by such methods.
2. Description of the Related Art
Engine valves, which are used for opening or closing intake and exhaust paths of an automobile or a motorcycle engine, are strongly required to have seizure resistance, abrasion resistance, impact resistance, and the like, because the engine valves are used in a harsh environment. Thus, various techniques have conventionally been proposed that apply several coating methods to a surface of an engine valve made from titanium or titanium alloy, so as to form a hardened coating for surface protection. For example, Japanese Laid-Open Publication No. 10-238320 discloses a titanium alloy engine valve coated with a CrN coating by a physical vapor deposition (PVD) process. Japanese Laid-Open Publication No. 1-96407 discloses a titanium alloy engine valve coated with a nickel alloy coating by an electroless plating process.
Forming such a coating on the surface of an engine valve may improve the abrasion resistance at a stem or a stem end portion of the engine valve. Especially in the case of a coating treatment by the PVD process, since the PVD process enables the coating to be extremely high in hardness, an engine valve coated by the PVD process may be drastically improved in abrasion resistance.
However, a coating formed by the PVD process is insufficient in impact resistance, although excellent in abrasion resistance. Thus, the extremely high hardness of the PVD coating on one hand results in drastically improving the abrasion resistance, but on the other hand results in poor adhesion between the coating and the substrate of the coating target. This is why an impact may cause the coating to be peeled off from the substrate.